1. Field of the Invention
The present invention relates to a pressure sensitive adhesive sheet. More particularly, the present invention relates to a pressure sensitive adhesive sheet which, at the time of working of the back of an adherend having a surface whose unevenness (irregularity) height differences are large, is preferably stuck to the adherend surface to thereby protect it during the working.
2. Description of Related Art
In the step of grinding the back of a semiconductor wafer, the surface thereof having electrical circuits formed thereon is protected by a pressure sensitive adhesive sheet. The height differences of customary circuits attributed to electrode elements have ranged from about 5 to 20 μm. For the wafer having these customary circuits formed thereon, the conventional surface protective sheets can fully protect the circuits without the occurrence of circuit breakage or wafer cracking and thus have been satisfactory.
However, in recent years, the method of mounting IC chips has become diversified. For example, a packaging method has been developed in which an IC chip circuit surface is arranged in a downward direction. In this packaging method, electrode elements protrude from the circuit surface and the height differences thereof are 30 μm or more, occasionally greater than 100 μm. Protrudent portions formed on the surface of semiconductor wafers like those mentioned above are referred to in the art as bumps.
At least two bumps are generally formed per chip. When the number of bumps is large, the bump pitch (distance between bumps) may be hundreds of microns. Depending on bump pitch pattern and chip arrangement, there occur dense portions where the bumps gather and portions where the bump pattern is sparse. In particular, the periphery of wafers is likely to become sparse in bump portions because of the absence of chips. With respect to wafers having the pressure sensitive adhesive sheet stuck thereto, the thicknesses thereof at dense bump sites and sparse bump sites conspicuously differ. The grinding of such wafers having conspicuous thickness differences would cause a problem because the thickness differences lead to a difference in the ground wafer thickness.
When the back of a wafer carrying such bumps is subjected to grinding while the wafer surface is protected with a conventional surface protective sheet, the wafer back is ground deeply in conformity with the configuration of the bumps which forms recesses (dimples) on the back of the wafer with the result that the thickness of the wafer becomes nonuniform. Further, cracking has been encountered, which occurs from dimple portions, resulting in breakage of the wafer.
A similar cracking problem is encountered with respect to the ink marking of failure circuits (bad mark) after the inspection of the wafer circuits.
For semiconductor wafers having large bumps, preventative measures have been taken which include, for example, decreasing the hardness of the substrate film of the surface protective sheet or increasing the thickness of the pressure sensitive adhesive sheet. These measures, however, have been unsatisfactory, and the above problems have not yet been resolved.
The present invention overcomes the problems found in the prior art and provides a pressure sensitive adhesive sheet which, at the time of working of the back of an adherend having a surface whose uneven height differences are large, is preferably stuck to the adherend surface to thereby protect it during the working. In particular, the present invention provides a pressure sensitive adhesive sheet which, when the adherend is to be ground to an extremely small thickness, enables performing the grinding at a uniform thickness without the occurrence of dimples.